The present invention relates to a method for operating an exhaust gas aftertreatment system of a motor vehicle, wherein the exhaust gas aftertreatment system comprises at least one NOx storage catalyst and at least one SCR catalyst. Furthermore, the invention relates to a computer program, a machine-readable storage medium and an electronic control device, which are provided for carrying out the method.
In order to reduce the pollutant concentrations in the exhaust gas of the internal combustion engine of a motor vehicle, use is made of complex exhaust gas aftertreatment systems. To reduce nitrogen oxygen emissions, it is already known to combine a Nitrogen oxide Storage Catalyst (NSC) with a SCR catalyst (Selective Catalytic Reduction). The nitrogen-oxide-reducing effect of the NSC is based on NOx (NOx—nitrogen oxides) being stored in the regular lean-burn mode of the engine. The NSC is regenerated by an intermittent provision of reducing agents in the exhaust gas, as a result of which the stored NOx can be reduced. This can take place in particular by means of a rich-burn mode of the engine (λ<1), wherein, after such a regeneration, the NSC is again capable of absorbing NOx.
Since the rich-burn mode carried out during the regeneration of the NSC increases the fuel requirement, the regeneration function for the NSC is generally applied by the vehicle manufacturer in such a manner that the regeneration is carried out as rarely as possible, but nevertheless as often as required, in order to be able to keep to the predetermined NOx limit values. The regeneration time is made dependent on higher-ranking criteria, such as, for example, the NOx charging of the NSC, and is therefore generally not variable in terms of time within a predetermined driving cycle having the same starting conditions since this would lead either to an increased fuel consumption or to higher NOx emissions.
The basic principle of an SCR catalyst consists in reducing nitrogen oxide molecules on the catalyst surface in the presence of ammonia (NH3) as reducing agent to form elemental nitrogen. The required reducing agent is introduced here, for example in the form of an aqueous urea solution, into the exhaust tract upstream of the SCR catalyst by a metering device. The required metering rate is determined in an electronic control unit in which strategies for the operation and the monitoring of the SCR system are generally stored.
In the case of a combination of an NSC and an SCR catalyst in an exhaust gas aftertreatment system, the emission load can be distributed to the NSC and to the SCR catalyst in a temperature-dependent manner. The NSC primarily takes on the reduction of the nitrogen oxide concentration here after a cold start, since, after a cold start, the SCR catalyst is generally still too cold to achieve good NOx conversion. If the SCR catalyst has reached its operating temperature, and a sufficiently large SCR catalyst is installed, the SCR catalyst can achieve the required NOx conversion rate by itself. In this case, regeneration of the NSC can be switched off in order thereby to save fuel.
Legal regulations in the sphere of the diagnostic analysis of emission-relevant components require, within the scope of what is referred to as on-board diagnosis (OBD), the monitoring of all exhaust gas aftertreatment components and of the sensor arrangement used in respect of maintaining the OBD limit values for the NOx emissions of the motor vehicle. OBDII legislation requires individual component monitoring in this case, and therefore each individual catalyst has to be monitored for the maintaining of the OBD limit value. It is therefore required for a possibly defective catalyst within a system to be unambiguously identified.